Printed circuits widely used in electronics and electrical devices, are formed with major dimensions of length and width and contain one or more circuits. The thickness of the printed circuit panels varies for many different reasons and directly affects panel flexibility, i.e., there are rigid and flexible circuit boards.
For example, printed circuits with multiple conductor planes use a bonded dielectric layer or layers for separation. Holes perforated through the circuit substrate serve a number of purposes including solder terminals for installation into another assembly, plated through hole interconnections between conductor planes and tooling registration holes.
Exposed copper on the finished printed circuit panels must, with few exceptions, be solder coated, a process sometimes termed presoldering or soldering. It is preferable that the solder coating be applied only where needed later and not on all conductor runs. To apply solder selectively, a dielectric, referred to as a cover dielectric or solder mask, is used to cover copper, the normally used conductor, that need not be solder coated. Thus desired copper portions remain exposed and are solder coated, including terminal pads and the like. In other words, the exposed copper on the surface or surfaces of the printed circuit must be effectively solder coated.
It is also necessary that all holes through the printed circuit be lined with solder but unobstructed by solder when finished so as not to obstruct later insertion requirements. The process of removing excess surface solder and clearing the holes of solder, after solder coating the surface conductors, is referred to as solder leveling or leveling.
In other words, printed circuit panels are soldered, inter alia, to maintain solderability for subsequent operations. For economic purposes, such soldering should be accomplished as a mass panel coating technique and should provide even coatings on the surfaces and in holes without surface flaws.
An improved apparatus for soldering printed circuit panels disclosed in my U.S. Pat. No. 4,608,941 for "Apparatus for Soldering Printed Circuit Panels" includes a roll configuration to convey the panels horizontally across a container of molten solder. Immersion of the circuit panels in flowing solder is followed by subjecting the panels to suitably positioned air knives for leveling the solder on the panels.
While the patented apparatus has provided significant advantages for soldering printed circuit panels, there remained the need for a soldering system that improves the speed and reliability of soldering printed circuit panels, and that can be operated over extended time periods with improved removal of dross and other impurities from the soldering system.
Moreover, following soldering and leveling of the printed circuit panels, the prior apparatus engaged and tended to disturb the still hot and unsolidified solder on the lower side of the printed circuit panels. It would be advantageous to convey the panels through a cooling system to solidify the solder without tracking or marring the wet soldered surfaces.